Aluminum is typically produced by the Hall-Heroult electrolytic reduction process wherein aluminum oxide dissolved in molten cryolite is electrolyzed at a temperature of from 900.degree. C. to 1000.degree. C. The process is conducted in a pot-type reduction cell which typically comprises an insulated steel shell lined with carbon or other refractory materials to contain the molten constituents. Iron conductor bars connected to a source of direct current are typically imbedded in the carbon lining comprising the floor of the cell and a carbon anode is suspended in the cell. Molten aluminum is electrolyzed out of the aluminum oxide-cryolite melt and collects on the carbon floor of the cell and is continuously or periodically withdrawn. A layer or pad of molten aluminum is maintained on the carbon floor of the cell which molten aluminum and carbon floor function as a cathodic surface.
To minimize voltage drop and optimize cell efficiency, the gap between the anode and the surface of the aluminum pad should be maintained as small as possible, preferably not more than about 3 centimeters. But this desirably close anode-cathode spacing is difficult to maintain due to magnetic induction currents which cause large perturbations in the molten aluminum pad which increase the risk of short circuiting the system by contact between the molten aluminum and the anode. For example, in a typical cell, the spacing between the anode and the surface of the molten aluminum pad cannot as a practical matter, be maintained at less than about 4 centimeters.
One means of overcoming this problem is disclosed in U.S. Pat. No. 4,071,420 wherein an array of cathode elements in the form of hollow bodies or tubular elements filled with molten aluminum protrude up through the aluminum pad and extend into the cryolite layer and terminate proximate the anode.
This arrangement has the effect of removing the region of electrolytic activity from the surface of the aluminum pad to the surfaces of the elements and their contained aluminum pools confronting the anode. Although this construction reduces the perturbative effect of magnetic induction currents and enables more precise control of the anode-cathode gap, i.e. 2 centimeters or less particles of undissolved bath materials or sludge tend to settle out in the bottom of the hollow bodies. Sludge accumulation in the interior of the hollow bodies is particularly disadvantageous when the hollow bodies are formed of an electrically non-conductive material since the sludge layer could act as an insulator and could disrupt the flow of electrical current from the carbon floor to the molten aluminum overlying the sludge layer.
Even in the case when the hollow bodies are formed of electrically conductive material, over an extended operating period sludge could build-up so as to substantially or completely fill the hollow body thus significantly impairing its ability to efficiently function as a cathodically active surface.